Some of the present inventors have previously proposed a process for catalytically cracking vacuum residual oil and other heavy hydrocarbons into light oils in the presence of a laterite catalyst, wherein the coke deposited on the catalyst is combusted in a reducing atmosphere to reduce the iron oxide in the catalyst, and then the reduced catalyst is contacted with steam to produce hydrogen (see U.S. Pat. Nos. 4,224,140 and 4,298,460).
As a result of further extensive study on the foregoing process, it has been found that when a catalyst containing iron in a reduced state is used, a reaction involving cracking of heavy hydrocarbons into cracked gases and light oils and coking and a steam-iron reaction of iron in a reduced state and steam simultaneously proceeds even at relatively low temperature, e.g., 500.degree. to 600.degree. C.
On the basis of the foregoing findings, some of the inventors further proposed a simplified process for simultaneously cracking heavy hydrocarbons into cracked gases and light oils and producing hydrogen, comprising a first step wherein steam and heavy hydrocarbons are simultaneously contacted with a catalyst containing iron in a reduced state to produce hydrogen, cracked gases, and cracked light oils and at the same time, to effect oxidation of the iron in a reduced state, contained in the catalyst and deposition of coke on the catalyst, and a second step wherein the catalyst containing iron in an oxidized state, with the coke deposited thereon, is contacted with an oxygen-containing gas insufficient for achieving complete combustion of the coke, to partially combust the coke on the catalyst and simultaneously reduce the oxidized iron in the catalyst, thus regenerating the catalyst (see U.S. Pat. No. 4,325,812, filed Sept. 29, 1980).
This process is hereinafter explained in detail.
This catalyst used in the process can be prepared by pulverizing, kneading, granulating and calcining materials having a very high iron content, such as natural ores containing iron in the form of hydroxide, oxide or carbonate (e.g., laterite, magnetite, siderite, limonite and hematite), or chemical compositions of iron oxides, hydroxides or carbonates, or mixtures thereof with inorganic materials, such as alumina, silica, magnesia, calcium hydroxide, and nickel oxide, and natural ores, e.g., garnierite, dolomite and limestone. To have high catalytic activity, these catalyst particles preferably have a specific surface area of from 0.1 to 30 m.sup.2 /g, and for use in a fluidized bed, the catalyst particles preferably have a mean diameter of from 60 to 600.mu.. When the iron (Fe) content of the catalyst is below 30% by weight, only a small amount of hydrogen can be produced. It is, therefore, essential for the catalyst to contain at least 30% by weight Fe. The Fe content preferably does not exceed 70% by weight because otherwise bogging (i.e., sintering and agglomeration of catalyst particles) occurs during the reduction of the catalyst in the second step.
The terms "in an oxidized state" and "in a reduced state" are used herein as a relative measure of the degree of reduction of iron oxides in the catalyst particles. The degree of reduction (R) of the catalyst particles can be represented by the following formula: ##EQU1##
If R.sub.1, which is the degree of reduction of the catalyst at the end of the first step, is smaller than R.sub.2, which is the degree of reduction of the catalyst at the end of the second step, the invention achieves its object, and the greater the difference between R.sub.2 and R.sub.1, the greater the amount of hydrogen that will be generated per given amount of catalyst recycled to the first step. In this process, therefore, a catalyst having a degree of reduction of R.sub.1 is referred to as a catalyst in an oxidized state (or as an oxidized-state catalyst), whereas the catalyst having a degree of reduction of R.sub.2 is referred to as a catalyst in a reduced state (or as a reduced-state catalyst). Both R.sub.1 and R.sub.2 are preferably greater than 11.1%; R.sub.2 is preferably in the range of from 12 to 50%, and more preferably in the range of from 15 to 40%.
Hereinafter the process will be explained in further detail with reference to the accompanying drawing.
In the first step 1 of the process, a catalyst bed 2 composed of catalyst particles in a reduced state, which are introduced thereinto through a catalyst transfer pipe 6, is maintained at a temperature of from 500.degree. to 800.degree. C. and a pressure (gauge pressure) of from 0 to 15 kg/cm.sup.2, and steam 3 and heavy hydrocarbons 4 are introduced into the catalyst bed 2 from the bottom thereof to form a fluidized bed where the steam-iron reaction (a reaction to generate hydrogen by contacting steam with iron (Fe) or iron oxide (FeO)) and simultaneously cracking heavy hydrocarbons are performed.
Heavy hydrocarbons which can be cracked to light oils with advantage by the process include relatively inexpensive high-boiling residual oils having more than 10% by weight Conradson carbon such as atmospheric residual oil, vacuum residual oil, solvent-deasphalting residues, tar sand, and shale oil.
The steam introduced into the catalyst bed causes the steam-iron reaction with the iron in a reduced state (i.e., Fe or FeO), contained in the catalyst, generating hydrogen and at the same time, oxidizing the iron in the catalyst into iron oxides (i.e., FeO or Fe.sub.3 O.sub.4). The heavy hydrocarbons are catalytically cracked into cracked gases, cracked light oils and coke. The thus-formed coke deposits on the surface of the catalyst particles.
About 30 to 50% of the sulfur contained in the heavy hydrocarbons is distributed into the coke. The amount of the coke deposited on the catalyst increases with the content of Conradson carbon in the heavy hydrocarbons and with the progress of the cracking reaction. For the purpose of the process, the amount of the coke deposited is preferably from 2 to 15% by weight, and more preferably from 4 to 12% by weight, based on the weight of the catalyst. If the amount of the coke deposited is too small, the iron oxides in the catalyst are not reduced adequately in the second step 8, and to provide a good heat balance for the reaction system, additional heat must be supplied in great quantity from an external source. On the other hand, if the amount of the coke deposited on the catalyst is too high, the catalytic activity of the catalyst is decreased and the reaction between the steam and the reduced-state iron is inhibited to decrease the amount of hydrogen generated.
Catalyst in a reduced state is converted into catalyst in an oxidized state by the steam. At this time, a part of iron sulfide present in the catalyst, i.e., iron sulfide resulting from the reaction of sulfur with Fe, FeO or Fe.sub.3 O.sub.4 at the second step 8, is converted into iron oxides and hydrogen sulfide. It is, however, not desirable from an economic standpoint to decompose all the iron sulfide resulting from the reaction because to do so requires a large quantity of steam. The coke on the catalyst reacts with steam to a slight extent to generate hydrogen, carbon monoxide, and carbon dioxide, but this reaction is insignificant at a temperature lower than 800.degree. C.
The amount of heavy hydrocarbons supplied to the first step is properly determined depending on the amount of coke deposited and the amount of catalyst particles recycled to the first step, and the amount of steam supplied is properly selected depending on the degree of reduction of the catalyst, the desired amount of hydrogen generated, and the amount of catalyst particles recycled to the first step.
Hydrogen, hydrogen sulfide, cracked gases such as methane, ethane, ethylene, C.sub.3 fractions, and C.sub.4 fractions, and cracked oils generated in the first step, and uncracked heavy hydrocarbons, steam and so forth are discharged from the top 5 of the first step 1 in the form of a gas and vapor. The thus discharged gas is introduced into a scrubbing column or a distillation column (not shown) where it is first separated into uncracked heavy hydrocarbons and cracked products. The uncracked heavy hydrocarbons are returned and mixed with a feedstock, heavy hydrocarbons, and again subjected to the cracking reaction. The hydrogen, cracked gases, cracked light oil and so forth are separated and recovered by known refining steps as a gaseous mixture which contains hydrogen as the major component, and, additionally, methane, ethane, ethylene, C.sub.3 fractions, C.sub.4 fractions, and hydrogen sulfide (H.sub.2 S). This gaseous mixture can be separated into high purity hydrogen and hydrogen sulfide, and propane, butane and other light hydrocarbon gases by suitable means, such as distillation, an amine absorption method, a pressure swing adsorption method [PSA method (see, for example, CEP, January (1976), pp. 44-49)] and a cryogenic processing [see, for example, CEP, September (1969), pp. 78-83], which may be used alone or in combination with each other. For example, the mixture can first be freed of C.sub.3 and C.sub.4 fractions by distillation or absorption, then freed of H.sub.2 S by amine absorption, and then freed of hydrogen and light hydrocarbon gases such as methane, ethane, and ethylene by the pressure swing adsorption method or cryogenic processing.
The cracked light oil is fed to the refining step where it is separated into a naphtha fraction, a kerosine fraction, a gas oil fraction, etc., which are desulfurized in a desulfurization step to provide the desired end products.
The oxidized-state catalyst with the coke formed in the first step deposited thereon is transferred to a second step 8 through a transfer pipe 7. A catalyst bed 9 is maintained in a fluidized state by introducing thereinto an oxygen-containing gas 10, e.g., air. The coke deposited on the catalyst is partially combusted with oxygen insufficient for complete combustion of the coke. The catalyst bed 9 is maintained at a temperature of from 750.degree. to 950.degree. C. and a pressure of from 0 to 15 kg/cm.sup.2 G, and is held in a reducing atmosphere produced by CO gas generated by partial combustion of the coke. The amount of oxygen in the oxygen-containing gas is preferably such that the mole ratio of oxygen supplied to coke deposited on the catalyst (coke supplied from the transfer pipe 7) (O.sub.2 /C) is from 0.1 to 0.6, with the range of from 0.2 to 0.4 being particularly preferred.
The coke deposited and CO gas generated by partial combustion of coke react with the catalyst in an oxidized state in the catalyst to form the catalyst in a reduced state. That is, the iron oxide in the catalyst is reduced. The residence time of each of the catalyst and gas, the amount of coke deposited, the amount of oxygen being introduced, and so forth are properly determined so that the degree of reduction of the catalyst is maintained in the above described range of from 12 to 50%.
The coke deposited on the catalyst reacts with the oxygen in the oxygen-containing gas and the oxygen in the catalyst to form gases such as carbon monoxide (CO) and carbon dioxide (CO.sub.2). Simultaneously, sulfur compounds contained in the coke are gasified into H.sub.2 S, COS, SO.sub.x, etc. These gaseous sulfur compounds are captured by Fe, FeO and Fe.sub.3 O.sub.4 in the reduced-state catalyst and are converted into iron sulfide. Therefore, the sulfur content of a large quantity of waste gas 11 discharged from the second step can be significantly reduced.
As the reaction temperature is increased (e.g., more than 750.degree. C.), the above described reactions such as partial combustion of coke, reduction of catalyst, and capture of sulfur proceed faster. It is preferred, however, that the reaction temperature does not exceed 950.degree. C., because otherwise bogging occurs among catalyst particles, undesirably making the fluidization and transfer of catalyst to another step difficult. It is, therefore, preferred to perform the reaction at a temperature between 750.degree. C. and 950.degree. C. The temperature can be maintained in this range by the partial combustion of coke. It is to be understood that to avoid the possibility that the heat balance in the reaction system may be upset, the system may be cooled or heated by a conventional external or internal means. For example, cooling can be achieved simply using a boiler type fluidized bed, and for heating, it is convenient to use a method in which torch oil (which may be the same as or different from the heavy hydrocarbon feedstock) is supplied directly to the catalyst bed 9. The amount of heat generated in the reaction system can also be controlled by changing the amount of oxygen in the oxygen-containing gas 10, although care must be taken so that the mole ratio of O.sub.2 /C is kept within the above defined range.
The waste gas 11 resulting from the reduction reaction and partial combustion of coke is generally discharged from step 8, e.g, after recovery of differential pressure energy by means such as a conventional gas expander, and of heat energy by means such as a conventional CO boiler, it is discharged outside the reaction system. Irrespective of the fact that the coke having a high sulfur content is burned with air, for example, the sulfur content of the resulting waste gas 11 is low, since the reduced-state catalyst in step 8 has the action of fixing sulfur compounds as iron sulfide. It is therefore possible to discharge the waste gas directly into the atmosphere without application of a desulfurization treatment.
The catalyst freed of coke and rendered to a reduced state is returned to the first step through a catalyst transfer pipe 6. In accordance with the process, while effecting reduction and regeneration of the catalyst simultaneously with combustion and gasification of coke in a high temperature reducing atmosphere, the sulfur compounds which are distributed in the coke during the cracking reaction of heavy hydrocarbons and the coking reaction, react with the iron in the catalyst, forming iron sulfide. It has been found, however, that when the catalyst is used while recycling for a long period of time, iron sulfide gradually accumulates in the catalyst, causing the problem that the amount of hydrogen generated is reduced.
In order to prevent the accumulation of iron sulfide, it has been proposed to use a method of converting iron sulfide into hydrogen sulfide and iron oxide with steam. This method, however, is not always economically advantageous because it needs a large amount of steam because of the equilibrium of reaction.